Lately I've been telling my students that there really is no secret
to aikido other than this:

Walk through doors, but don't walk through walls.

Now, I really believe this. It encapsulates the essence of
non-resistance, it illustrates the hard and the soft of yin and yang,
the balance of freedom and restriction. It addresses almost all the
problems we encounter in our training, and simplifies the analytical
process and corrective approach.

Let's look closer at the analogy. In aikido, both the attacker and
defender are looking for openings (doorways). Both players are
negotiating resistances and obstacles (walls). A doorway is any
passage around a person or body part. A wall is any part of the body
that blocks passage. A door is a body part that can be moved in such
a way as to create an opening. Sometimes the whole body can be a
door.

Specifically, the doorways that we should be aware of are the open
spaces between the head and shoulders, between the extended arms and
the torso, and less commonly, between the legs. Again, the whole body
can be moved, creating space for flow to occur. Or, just the upper
body can be tilted aside, as with koshi-nage. To escape from a
hand grab, we learn to utilize the door of the thumb, and not collide
with the walls of the fingers.

We often duck under our partner's arms. In kids' classes especially, I
find it useful to describe this as going through the door. From
katatedori, tell a kid to go through the front door and watch
as ikkyo, sankyo, or even kaiten nage
spontaneously emerge. Tell them to go through the back door, and
shihonage practically does itself.

Once we have learned to identify the walls and doors, we can easily
avoid needless resistance and move toward safe, open passage. In
theory, nothing could be simpler. We open a door for our attacker and
escort them safely through. We find doors within the combined form of
our bodies, and we enter (irimi) or pass through
(sudori). We practice tenkan and we become the door.

Last night they loved you, opening doors and pulling some strings

~ David Bowie "Golden Years"

Like I said, it's really quite simple. In practice, however, we
should expect things to be a bit more challenging.

So, extending the analogy, we need to consider the following:

Some doors may be hidden, some doors may be traps

Some doors have handles and latches

Some doors may be stuck, or may be very heavy

A door that is locked will first need to be unlocked

Locks require keys, or lockpicks

A door that may not be unlocked is really a wall

Some rooms have no doors.

So, we seek non-resistance in our aikido. If there is an open door
that we know we can safely pass through that leads to a favorable
place, we should go there rather than through more difficult
passages. But free passage may not always be available at every
moment. So the following observations result from the above
considerations:

Appropriate doors must be discovered or created

Handles may need to be turned, latches may need to be pressed

Stuck doors may need to be unstuck, heavy doors may require more effort

Locked doors will need to be unlocked

Not all doors can be unlocked, even with effort

Creating a door in a wall may require violent force.

And of course, the architecture of aikido attack and defense is often
more dynamic than static. It is often the case that the doors will
pass around you as you stand still. Here, the idea of moving through
a door is relative.

A door is what a dog is perpetually on the wrong side of.

~ Ogden Nash

My goal is to move through life as fluidly as possible. I may embrace
resistance and challenge for fun and discovery, but even then I wish
to eliminate all violence and wasted energy as much as possible.

But if ever trapped in a burning building, I don't want to hesitate
to put a chair through a window, take a hammer to a doorknob, or a
table leg to the sheetrock.

Yet neither do I want, in my panic, to overlook the fact that the
fire itself may have opened a new exit for me.

And in any case, I wouldn't want to apply emergency escape measures
in places where they are unnecessary and destructive. In other words,
we want to know what to do in an crisis, but we don't want to make a
way of life from it.

After all, it's the nature of our practice that determines the
quality of our passage through life.

There are things known and there are things unknown, and in between
are the doors.